Function hydrateShadow

The client half of banira's Declarative Shadow DOM prerender (see createPrerenderer/banira prerender). Call it from a custom element's constructor or connectedCallback:

• If the parser already attached a non-empty shadow root (from DSD), it is adopted as-is — no re-render, so the prerendered markup never flashes.
• Otherwise a shadow root is created and template is rendered into it.

In both cases any styles are adopted, since constructable stylesheets are not part of the serialized DSD markup. Returns the shadow root and whether it was hydrated, so callers can skip re-binding already-bound prerendered nodes.

connectedCallback() {
  const { shadow, hydrated } = hydrateShadow(this, { template: TEMPLATE, styles: sheet });
  if (!hydrated) this.wireUpFreshDom(shadow);
}
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